Study On Microseisms Based On Seismic Array Technology

Seismic ambient noise is generally defined as the continuous background record on the seismogram and it is caused by multifarious persistent faint vibration of the earth that is not related to an earthquake.Global seismic background noise models show especially strong amplitudes at periods of 1-20 s.The energy in this period band is referred to as microseisms and it is generally created by the nonlinear interaction of ocean waves and solid earth.Additionally,in the latest ten years ambient noise tomography(ANT)also focus on the microseism period,which has been widely applied in studying the structure of the earth interior.The theory behind this technique presupposes a diffuse symmetric noise field.In practice,however,anisotropy usually exists in the noise,resulting in the bias of Green's functions.The biasing problem could be mitigated by determining more precise locations of the microseismic sources as well as better understanding of the composition and its generation mechanism of the microseism filed.Consequently,the study on source location,composition and generation mechanism of microseisms,especially the transversely polarized components plays an important role on imaging the earth's interior structure using Rayleigh,Love and even body ambient noise tomography.Not only Rayleigh,Love waves but also teleseismic P wave and local S wave in microseisms have been definitively observed in ambient noise.The models that ocean waves excite microseismic energy mainly produce a near-vertical single force which is difficult to effectively generate transversely polarized microseisms(e.g.Love wave and SH wave).The excitation mechanism of the transversely polarized microseisms has been in great debate.Teleseismic SH wave has been taken as an ideal method to study these transversely polarized microseisms because its Green's function is relatively simple and the propagation path is less affected by the 3-D mantle structure.However,persistent teleseismic SH waves are not widely observed in the world.Further research work should be done to obtain accurate and robust teleseismic SH microseism sources,which in turn,contribute to better understanding the origin of transversely polarized microseisms.In this study,we carry out a global survey of ambient seismic noise at period of 1-20s based on the three component continuous seismogram recorded by a large-aperture array(Wutan Array,WTA)deployed in China during 2013-2014.To locate microseismic sources of SH waves(as well as SV and P waves)we continuously backproject one year of vertical,radial,and transverse components of the ambient seismic wavefiled recorded by WTA.To the best of our knowledge,this is the first study that identifies the source regions of teleseismic S waves observed in microseisms.And the results from back projection have been confirmed by conventional three-component frequency-wavenumber(f-k)processing.At the same time,we study on the surface microseism based on f-k analysis.Then we give the composition of microseisms and its source locations distribution in space and time.At last we document the possible excitation mechanisms of the different microseisms especially the transversely polarized energy such as teleseismic SH and Love wave taking into account both of the seismic observation and the predictions from the IFREMER ocean wave model that is built within the WAVEWATCH III framework.It is indicated by the results that persistent and robust microseismic sources of SH waves(as well as SV and P waves)are observed in the North Atlantic,North Pacific,and Indian Oceans at periods of 2.5-20 s,in regions with the strong ocean wave interaction.P waves have larger amplitude than SV waves,which are larger than SH waves.P and SV waves originate from the same areas,SH waves from slightly different areas.In addition,vertical f-k analysis show that Rayleigh wave microseisms at period of 10-20 s are detected by WTA which originate from the North Atlantic,West Pacific and Indian Oceans.Love wave microseisms are identified on the transverse component while Rayleigh wave are on both of vertical and radial components,according to the test results from three-component f-k analysis using one of subarrays of WTA.And the Love and Rayleigh wave microseisms display similar back azimuths in spite of they appear on different components.Similarly,they also come from the orientation of North Atlantic,West Pacific and Indian Oceans.Observations of WTA show a seasonal variability of teleseismic body wave source areas and direction distribution of surface waves in microseisms.In northern winter,the North Atlantic and West Pacific are the two strong microseism source regions while in northern summer,the intense wave activity across the South Pacific and the Indian Ocean dominates the microseism generation.As to the frequency dependence of the microseisms source distribution,there is no obvious difference for teleseismic body wave at the three separate period bands but with a certain discrepancy about the energy and resolution.Compared with body wave,the azimuth distribution of the Rayleigh surface wave displays obvious frequency dependence.That is,the dominant direction of Rayleigh wave is NNW to the WTA location which is the orientation of the North Atlantic at the relatively short period of 12-20 s while it turn to SE direction(also to WTA),the West Pacific region,at the shorter period of 9-12 s.The seasonality and frequency dependence of microseisms sources distribution both for surface and body wave are due to seasonal variation and the spectrum characteristic of the ocean activity itself.Taking into account of observations as well as its coherence with the predictions from the IFREMER ocean wave model,we document the possible excitation mechanisms for different body waves as follows:(1)The teleseismic P wave are mainly in the secondary microseisms and created by the nonlinear interactions of the two nearly counter-propagating ocean waves in coastal and deep water regions.(2)The teleseismic SV signals that we observe on radial-component WTA array beams are considered to be generated by the superposition of multiply reflected P waves in the water column,which convert to SV waves at the interface between the ocean and the solid Earth.(3)The teleseismic SH waves that we observe on transverse component array beams are more difficult to account for.One possibility is scattering of the near source P-SV wavefield by small wavelength features such as seamounts or sedimentary basins.A second potential explanation for our observations of teleseismic SH waves is shear traction induced by the direct interaction of ocean waves with seafloor topography.The seismic array analysis also indicates that both surface and teleseismic body wave can be generated by some specific storms occurred in the West Pacific which is located in the southeast to the WTA array center.Specifically,the microseisms energy excited by the distant storms can propagate as Rayleigh,Lg,teleseismic P,SV,SH phases.A possible mechanism has been proposed that the ocean swell near storm center reach to coast after a long time propagation and then excited primary Rayleigh wave microseisms in the region of coast or continental shelf while the secondary Lg and teleseismic body wave(P,SV,SH)are due to the interaction between the concentric storm-generated wavefileld and the ambient monodirectional swell.Better understanding of the excitation mechanism should be based on the accurate and high-resolution microseisms locations.Therefor,further work should focus on how to improve the accuracy and resolution of the microseisms generation regions through multiple array beamforming constrain together with improvement of seismic array technology.Additionally,dense seismic array technology is a booming branch and it is extremely active in seismology research area.And the short period dense array detection is a newly developed technology of 21st Century,having been used to image the fine crustal structure.It has a very broad range of geophysics application including safely exploit of the urban underground space,the monitoring and investigating of the key area with earthquake hazards,urban active fault survey as well as mineral resources survey.In addition to the progress of the array technology itself,It is also a new development direction of array seismology that how to solve some scientific questions combining with other branches of geophysics,geology,oceanography.